Figure 1: The Pentacene Molecule

Organic semiconductors have gained a lot of interest during the last years, because of their usability for organic thin film transistor (OTFT) displays.

During the last 15 years, the mobility of electrons in these films, finally determining the speed of the displays, could be increased by more than a factor of 10000. This was achieved by selecting new organic materials (Fig.2). By now, Pentacene, a compound of Carbon and Hydrogen (C₂₂H₁₄) seems to be the most promising material for OTFT displays. A sketch of the molecule is shown in Fig.1. But not only the material chosen influences the final speed of the assembled transistors, but the quality and smoothness of the organic film turns out to be crucial. Pentacene molecules stand up vertical on the substrate, and the crystals they form can be rotated relative to each other by certain angles. Each boundary between two of these crystals forms a barrier for electrons, and therefore decreases the device speed. To understand the crystal growth of pentacene, the shape of the formed crystals, and to find supplementals that help to grow smooth and homogeneous pentacene films, a LEEM and PEEM study on the Pentacene growth on various surfaces was performed.

Figure 2: Development of electron mobility in organic transistors over time. After: Dimitrakopoulos et al. J. Appl. Phys.,80, 2501 (1996)

Pentacene growth on glass

Initially no contrast is visible in the movie - the clean glass surface appears homogeneously dark. After several minutes, first pentacene islands appear. These islands are extremely small, and with further deposition tend to gain height and grow in the third dimension. This results in a rough film with a huge number of grain boundaries between the crystals and a poor film quality if one thinks of device applications.  

The question is, though, whether the growth of a poor film is related to the properties of the pentacene (that does not want to grow as a smooth layer), or whether glass does not provide a suitable substrate for pentacene film growth. 

Initially no contrast is visible in the movie, and the silicon surface appears as a homogeneous gray area.  After several minutes of deposition, Si crystals are formed, and grow slowly into the field of view. Compared to the glass case, the pentacene islands are huge, with a size of more than 30µm. But, even more interesting, the shape of the islands is changed into a fractal landscape.

Compared to the glass movie, where 3 dimensional growth took place at defects,  here a nice layer by layer growth is visible and allows an observation of the electronic (band) structure of the film.

First Layer: The pentacene layer appears brighter than the silicon substrate. The number of electrons available for excitation by the mercury light is higher in the first pentacene layer than it is in the silicon "bulk" crystal.	Second Layer: The second layer is still brighter than the silicon substrate, but is darker than the first layer. This means, that the electronic structure from the first to the second layer is changed in a way, that fewer electrons can be excited by the mercury light.	Third Layer: The third layer is darker than the first and second layer, but the contrast is weaker. With increasing film thickness, the electronic properties of the band structure develop towards the expected properties for the perfect bulk pentacene single crystal.